1. Field of the Invention
The present invention relates to a compensating method and a compensating apparatus for a positioner, which make it possible to correct the temperature characteristic and the nonlinearity of a position sensor and obtain an output involving few errors.
2. Description of the Related Art
A positioner has been hitherto used to control a pressure fluid-driven apparatus such as a diaphragm motor and a cylinder. Such a positioner controls the pressure fluid on the basis of a difference signal between an input signal as a control command signal given, for example, as an electric signal, a pneumatic pressure signal or the like, and an output signal of a position sensor for detecting, for example, the displacement of an output shaft of a diaphragm motor or the position of displacement of a piston of a cylinder so that the output shaft, the piston or other components may be displaced to an angle or a position based on the control command signal. In the case of such a positioner, for example, when an MR element (magneto resistive effective element) is used for the position sensor, the service live of the position sensor is prolonged, because there is no sliding section. Further, it is possible to make highly accurate control, because there is no noise in output signal, and the resolution is enhanced.
However, the output signal of the position sensor such as the MR element greatly depends on the environmental temperature, and it has a nonlinear output characteristic with respect to, for example, the position of the output shaft of the diaphragm and the displacement position of the piston. Therefore, it is necessary to use a temperature-compensating circuit and a linearity-compensating circuit.
As a result, a problem arises in that the production cost of the positioner is expensive.
Further, "Electro-pneumatic converter calibration method" is disclosed in Japanese Laid-Open Patent Publication No. 6-207601. The method comprises the steps of calibration; a calibration sequence in which correction values for electrical signal inputs are obtained for a plurality of set temperatures and stored in a memory; a compensation sequence in which an active correction value is determined for a particular electrical signal input against the measured temperature and stored in the memory; and an operation sequence wherein the active correction value is read from the memory and applied to the electrical signal input to provide a corrected input signal for operating the electro-pneumatic converter device. Linearization method includes the steps of adjusting the positioner feedback linkage until a predetermined reference position is attained, reading a position feedback value from a feedback position sensor, and determining a position feedback linearization reference value by varying the positioner electrical signal input to match the positioner feedback linkage non-linearity, and storing the reference value in the memory.
In the above-mentioned method, however, compensation values over respective points for all the temperatures and all the positioner inputs should be stored in a memory, so that the number of data tends to extremely large. As a result, a problem arises in that a memory having mass storage is necessary.